Known downhole pulling tools are often fully mechanical tools, wherein a mechanical locking mechanism is used to provide a rigid connection between the pulling tool and an object to be retrieved downhole, such as downhole plugs, tools, etc. The mechanical locking mechanism often comprises locking means, such as keys, dogs or fingers, extending from the body of the pulling tool to engage a recess, also denoted as a fishing neck, in the object to be retrieved. The locking means are usually locked in the fishing neck by applying a pulling force to the pulling tool, and subsequently the object may be pulled out of the well.
Tools or plugs set in a well often deploy some kind of anchoring mechanism which has to be released before they can be pulled out of the well. This anchoring mechanism may be disengaged by applying a pulling or pushing force to the tool or plug whereby a shear pin is broken. When the shear pin breaks, the anchoring mechanism is automatically retrieved or disengaged. However, in some cases the anchoring mechanism may get stuck, or it might not be possible to pull the tool or plug out of the well for other reasons. In those cases the pulling tool requires a fail-safe mechanism ensuring that the connection between the pulling tool and the object to be retrieved can be detached downhole. In known pulling tools, such fail-safe mechanisms are often constituted by a shear pin or other locking member. If the pulling tool gets stuck, the shear pin may be broken by jarring down on the GS pulling tool, i.e. by applying a pushing force to the pulling tool, whereby the mechanical locking mechanism is disengaged and the pulling tool can be detached from the tool in the well and retrieved to the surface. One of the disadvantages of the known pulling tools is that the tools are damaged or become inoperable following the activation of the fail-safe mechanism.